This application seeks support for the acquisition of a VG Instruments OPTIMA stable isotope ratio mass spectrometer for analysis of H2, C02 and double labeled water. This system includes an automatic double trapping system with -196C micro cold finger, data system including Compaq 386 20E and state-of-the-art software, and a 50 sample breath carousel. The instrument would be part of the existing Shared-Instrument GC/MS Facility. As directed by the Mass Spectrometry Advisor Committee, the instruments would be available to all investigators at Harbor UCLA Medical Center and at a number of other institutions.